1. Field of the Invention
The present invention relates to an image sensing apparatus capable of sensing an image with a grid.
2. Related Background Art
When a certain fluorescent material is irradiated with radiation such as X-ray, α-ray, β-ray, γ-ray, electron beam or ultraviolet light, a part of the radiation energy is accumulated in the fluorescent material. It is already known that, by irradiating the fluorescent material with exciting light such as visible light, the fluorescent material shows exhaustive light emission according to the accumulated energy, and the fluorescent material showing-such property is called accumulating or exhaustive fluorescent material.
Conventionally there is already known a system of utilizing such accumulating fluorescent material, recording the radiation image information of an object such as human body on a sheet of the accumulating fluorescent material, then scanning the accumulating fluorescent sheet with exciting light such as laser beam to induce emission of the exhaustive florescent light and photoelectrically reading such light emission to obtain image signal. A radiation image information recording/reproducing system, for outputting the radiation image of the object on a recording material such as a photographic sensitive material and a display apparatus such as a cathode ray tube based on such image signal is disclosed for example in the Japanese Patent Application Laid-open No. 55-12429 and 56-11395, for example.
Also in recent years, there is being developed an apparatus for sensing the X-ray image utilizing a semiconductor sensor. In comparison with the radiographic system utilizing the conventional silver halide-based photography, such system has a practical advantage of recording the image over an extremely wide exposure range. Such system reads the X-ray of an extremely wide range with photoelectric conversion means and converts it into the electrical signal. It is thus rendered possible to obtain a visible radiation image not affected by change in the radiation dose, by outputting such electrical signal to a recording material such as a photographic recording material or a display apparatus such as a cathode ray tube.
In the analog image sensing with the above-described conventional accumulating fluorescent material, there are employed various grids for reducing the scattered ray at the image sensing, but, in the digital image data utilizing semiconductor sensor, there are generated Moire fringes and shading phenomenon because of the relationship between the sampling pitch and the grid frequency. One of the reasons causing such shading, in case of a moving grid, is the difference in the image collecting time at the gain image sensing and the object sensing, whereby the mode of grid movement is different to generate a special pattern.